Fishfinders as one type of transceiving devices of pulse signals transmit a transmission pulse of ultrasonic wave underwater from a transducer, and receives with the transducer echo signals produced by the transmission pulse, which are reflected by a seabed or a shoal of fish. The fishfinder then processes the received signal, and displays an image of the seabed or shoal of fish on a display thereof.
With such fishfinders, improvement in detection distance is required to detect a shoal of fish at a deeper location. In order to improve the detection distance, generally, the transmission pulse is simply lengthened. However, if the transmission pulse is lengthened, the reflected echo from a target object will also become longer and axial resolving power will be reduced. Therefore, a method is adopted in which a transmission pulse is frequency-modulated, and the echo signal received with a matched filter, and a replica waveform of the transmission signal are correlation-processed to pulse-compress the received signal to improve the axial resolving power. An underwater detection device that performs such a pulse-compression process is disclosed in JP2005-249398(A).
In the fishfinder that performs the pulse-compression process, false images referred to as “range side lobes” occur at positions before and after an echo that shows a location of a target object. When the range side lobes occur, it is difficult to distinguish a detected target image from the others, and particularly, range side lobes caused by the seabed hide an image of a shoal of fish located at the seabed and, thus, may make the shoal of fish undetectable. In order to suppress the range side lobes, generally, a measure of multiplying the transmission signal by a window function, such as a Gaussian window, is performed.
When applying the window function (e.g., Gaussian window) to the transmission signal to perform the matched filtering process, expansion of a dynamic range of the window function is effective to reduce the range side lobes. However, in order to realize a target suppression level for the range side lobes of less than −64 dB, which is required for typical fishfinders, a measure is required to acquire a dynamic range of 40 dB or greater and control B pressure of a PDM transmitting amplifier, etc. Thus, hardware thereof will be complicated and the cost will be increased as well.
Further, the multiplication of the transmission signal by the window function having an acute shape, such as a Gaussian window, substantially shortens a pulse width. Therefore, detection performances, such as a signal-to-noise (S/N) ratio and a detection distance, may deteriorate comparing with a case using a transmission signal having a rectangular envelope.